1. Field of the Invention
The present invention relates to image forming apparatuses used in a printer, a facsimile, a copying apparatus and the like employing electrophotographic system. More particularly, the present invention relates to an image forming apparatus by which single apparatus a black and white, multicolor, or bicolor image can be produced on a sheet of paper.
2. Description of the Background Art
The electrophotographic system to which Carlson process is applied is widely employed in conventional image forming apparatuses using toner. The principle thereof will be described by illustrating positive development system used in a copying apparatus or the like as an example. Referring to FIG. 13, an apparatus utilizing Carlson process includes a corona charger 2, an exposure device 3, a developing device 4, a transfer device 5, a fixing device 6, a cleaner 7, and a corona discharger 8 provided in this order around a photoreceptor drum 1 having a photosensitive layer on its surface.
In the structure above, the surface of photoreceptor drum 1 is uniformly charged by corona charger 2 in a dark place. By projecting an original image onto the surface of photoreceptor drum 1 by exposure device 3, charges are removed at the portion exposed to the light, thereby forming an electrostatic latent image on the surface of photoreceptor drum 1. Toner 9 charged to have the polarity opposite to that of charges on photoreceptor drum 1 is attached to the electrostatic latent image, so that a visible image is formed with toner 9. The visible image is superimposed on a sheet 10, and from the backside thereof charges with opposite polarity to toner 9 are applied by corona discharge using transfer device 5, thereby transferring a toner image to sheet 10. The transferred toner image is fixed to sheet 10 by means of heat, pressure and the like of fixing device 6 and becomes a permanent image. Residual toner 9a which has not been transferred to sheet 10 but remains on photoreceptor drum 1 is removed by cleaner 7. The electrostatic latent image on photoreceptor drum 1 is discharged by light emitted from corona discharger 8. Thereafter, the process starting from charging by corona charger 2 is repeated and images are formed successively.
In the electrophotographic system to which Carlson process is applied, a corona discharger is usually employed so as to charge photoreceptor drum 1 or to transfer toner 9 to sheet 10. Corona discharge requires application of a high voltage of several KV, and is easily influenced by environmental change, such as a change in temperature which brings about fluctuation in the amount of charges on the surface of photoreceptor drum 1. In addition, ozone is generated by corona discharge, giving rise to an environmental problem.
In view of the foregoing, an image formation method which does not require corona charge described above is disclosed in Japanese Patent Publication No. 2-4900. FIGS. 14A-14C show the image formation method disclosed in Japanese Patent Publication No. 2-4900. Referring to FIGS. 14A-14C, a photoreceptor 11 is formed by a transparent substrate 12 of glass or the like, a transparent conductive layer 13 of In.sub.2 O.sub.3 or the like, a photoconductive layer 14 of Se or the like, and an insulation layer 15 of polyethylene telephtalate film or the like, stacked in this order. When a magnet 17 having conductive magnetic toner 16 attached thereto and serving as a toner retainer is brought near the surface of photoreceptor 11, and light is directed from the side of transparent substrate 12 with a voltage applied between magnet 17 and transparent conductive layer 13, electric resistance of photoconductive layer 14 is reduced at the portion exposed to the light, and charges are introduced up to portion under insulation layer 15. As a result, a strong electric field is generated between magnet 17 and photoreceptor 11 and charges with the opposite polarity are introduced to a portion of toner 16 corresponding to the exposed region. Consequently, charges of toner 16 and charges introduced from transparent conductive layer 13 form charge pairs of opposite polarities, thereby attracting each other. Even if magnet 17 is moved away from photoreceptor 11 thereafter, only the portion of toner 16 corresponding to the exposed region remains on the surface of the photoreceptor.
In accordance with the principle above, a toner image can be formed on the surface of photoreceptor 11 without employing a corona charge method. After the toner image is formed on the surface of photoreceptor 11, it is transferred from the surface of photoreceptor 11 to the surface of a sheet (not shown). Toner is then heated, fused and fixed onto the sheet, on which an image can be formed as a permanent image.
An apparatus for forming a multicolor image which utilizes a principle similar to that in above-mentioned Japanese Patent Publication No. 2-4900 is disclosed in Japanese Patent Publication No. 60-59592. FIG. 15 is a schematic cross sectional view of an apparatus for forming an image with three kinds of toner, disclosed in the above publication. Referring to FIG. 15, toner 20, 21 and 22 having different colors, developing devices with a magnetic brush 26, 27 and 28, a back electrode 29, a light emitting diode 30, and a cleaner 32 are provided around photoreceptor 11. As photoreceptor 11 rotates, toner images 23, 24 and 25 are successively formed with toner 20, 21 and 22 at portions in contact with developing devices 26, 27 and 28 respectively, and three toner images 23, 24 and 25 formed on photoreceptor 11 are transferred to a sheet 31 and then fixed thereto.
FIG. 16 schematically shows a structure of another exemplary apparatus for forming a multicolor image which utilizes principle similar to Japanese Patent Publication No. 2-4900. Referring to FIG. 16, an apparatus 200 for forming a multicolor image includes a photoreceptor drum 201 and four developing devices 202-205 for multicolor printing, for example, provided around photoreceptor drum 201. In photoreceptor drum 201, an exposure device 206 is rotatably provided so as to take an appropriate position with respect to respective developing devices. Images are transferred color by color to a sheet 209 by a transfer roller 207, fixed by a fixing roller 208 and recorded on sheet 209. A mechanism is provided for moving the developing device not used for recording away from photoreceptor drum 201 in the directions indicated by allows in the figure. Sheet 209 is guided by a pull-in claw 210 and guide plates 211 and 212, and passes over transfer roller 207 the number of times required for printing. After the sheet is printed the required number of times, pull-in claw 210 is closed, so that sheet 209 is ejected.
Although the multicolor image formation apparatus described above can be made compact thanks to reduction in the number of components around photoreceptor drum 1 and it can produce multicolor, monocolor and black and white images, it has the following problems.
(1) When a multicolor image is to be formed, sheet 209 must move a plurality of times over transfer roller 207, and thus formation of an image requires a long time.
(2) The photoreceptor rotates and the outer peripheral surface thereof contacts the developing units for respective colors. As a result, developer (including toner) contacts the photoreceptor more than necessary, and toner attaches to a region which need not be developed, thereby generating a fog. In addition, when the toner on the development roller is isolated from the photoreceptor, it is blown off to inside of the apparatus by the wind generated by rotation of the photoreceptor, which is a main cause for scattering of toner.
(3) Even after an image is developed at a first developing unit, the photoreceptor is still in contact with developer thereof, whereby it requires cleaning after development. As a result, a desired hue cannot be obtained and the hue obtained at a second developing unit is a mixture of the original hue of second developer and that of first developer, leading to an undesirable change of the original hue of developer.
(4) The rotating photoreceptor degrades because of charge introduction and wear caused by the contact with developer. The toner on the photoreceptor adversely affects surrounding components due to a slight increase in temperature of the photoreceptor caused by a heat source for fixing, eventually resulting in heat history degradation of the photoreceptor. Since use of such a degraded photoreceptor adversely affects the image quality and the like, photoreceptors must be replaced frequently.
(5) Upon feeding a sheet to the photoreceptor, in order to align the leading edges of an image region and the sheet, timing of feeding the sheet must be accurately synchronized with rotation of the photoreceptor, which requires complicated control. Consequently, deviation of mispositioning of an image is very likely.
(6) Since mixing of developers is prevented by moving developing devices, a complicated structure is required.
(7) As a transfer roller for transferring an image from the photoreceptor to the sheet is provided, a transfer mechanism is needed. Therefore, control for transfer operation is required, resulting in complicated structure and an increase in cost.